This invention relates to a multi-piece hollow impeller and a method of manufacturing and using the same. The impeller is suitable for use in a radial flow centrifugal compressor, for example, or other rotary machines.
Small gas turbine compressors often use a radial compressor impeller as a last stage to boost air pressure. The radial compressor impeller includes a metal wheel with curved blades that accelerate the flow of air from an inlet near the inner diameter of the impeller to an exit near the outer diameter of the impeller. The impeller includes a single bore, or support structure, that carries the centrifugal loads on the impeller. The single radial impeller stage provides a pressure rise equivalent to the pressure ratio that several axial compressor stages can provide but with fewer parts. The single stage impeller also serves to reduce compressor axial length relative to axial compressor stages at an equivalent pressure rise.
Current impellers typically have an asymmetric solid, radar dish-shaped bore that tends to roll and deflect axially when under high centrifugal loads. In particular, conventional impellers axially deflect at the impeller tip in generally the opposite direction as airflow into the impeller inlet. The deflection is caused by centrifugal inertial loads on the asymmetric impeller and by temperature gradients in the impeller. As a result, the compressor must be designed with clearances to accommodate the deflection of the impeller tip throughout its entire operating range. The compressor is designed such that a desired clearance is obtained at a particular operating condition of the compressor, which results in less than desired performance during off design point operation reducing the overall efficiency of the compressor.
What is needed is an impeller that provides improved axial tip clearance throughout the entire operating range of the compressor.